A heat recoverable article such as a heat shrinkable tube has been used for various applications including processing of a joint (connected part) of electric cables and an end of wiring in various apparatus, and protection against corrosion of a steel pipe.
For example, a heat shrinkable tube is placed on a joint of electric cables and then heated, the tube is shrunk to fit in close contact along the contour of the joint, and the joint can be protected from external injury.
In the event that higher close contact is required to yield waterproofness, a heat shrinkable tube with an adhesive is formed by coating a hot-melt adhesive on an inner surface of a heat shrinkable tube.
As a resin used as the hot-melt adhesive, a hot-melt adhesive including an ethylene-vinyl acetate copolymer (EVA) and an ethylene-ethyl acrylate copolymer (EEA), and a hot-melt adhesive including a polyamide resin and a saturated copolymerization polyester resin have been generally used.
From these, a suitable hot-melt adhesive is selected depending on the material of the heat shrinkable tube, on which the adhesive is coated, and an adherend.
The following method is generally used as a process for producing a heat recoverable article, such as a heat shrinkable tube. A resin composition comprising a polymeric martial is shaped into a tube form, and crosslinked or partially crosslinked by irradiation of an ionization radiation, such as electron beams and .gamma.-ray, chemical crosslinking, or silane crosslinking. It is then inserted into a mold having a prescribed shape and expanded under a high-temperature atmosphere, followed by cooling while maintaining its expanded shape.
In the production process, the crosslinking with an ionization radiation has good productivity owing to a short crosslinking time, and thus is widely employed in industry as a crosslinking method of a heat shrinkable tube.
In the event that a hot-melt adhesive is used as an inner layer material, the hot-melt adhesive is previously coated on an inner surface of the heat shrinkable tube, which is then crosslinked by irradiating an ionization radiation.
A polyamide-based hot-melt adhesive, a saturated polyester-based hot-melt adhesive, an EVA-based hot-melt adhesive and an EEA-based hot-melt adhesive have been used as an inner layer of a heat shrinkable tube.
The polyamide-based hot-melt adhesive and the saturated polyethylene, polyester-based hot-melt adhesive can be extremely well adhered to a polar material, such as a polyvinyl chloride resin (PVC) and a metal, but cannot be strongly adhered to a non-polar material using a polyolefin, such as polyethylene, EVA and EEA.
On the other hand, the EVA-based hot-melt adhesive and the EEA-based hot-melt adhesive having a small vinyl acetate (VA) content and a small ethyl acrylate (EA) content, respectively, can be well adhered to a polyolefin such as polyethylene, EVA and EEA, but cannot be well adhered to PVC. Those having a relatively large VA content or EA content can be well adhered to PVC and a metal, but have a lower adhesive strength to a polyethylene resin (PE). The EVA and EEA resins having a large VA content or EA content necessarily have a low softening point, and have a drawback of being unable to be practically used because tackiness occurs in a room temperature range, and they are melted under a high-temperature atmosphere.
Even when the well adhered polyamide-based hot-melt adhesive to PVC and a metal and the well adhered EVA-based or EEA-based hot-melt adhesive to PE are simply mixed with each other, a hot-melt adhesive that can well adhere to all PVC, a metal, and a polyolefin such as PE and EVA cannot be obtained due to incompatibility with each other.
JP-A-7-157734 and JP-A-9-111215 disclose a hot-melt adhesive containing a polyamide resin, an EVA having a vinyl acetate content of from 20 to 60% by weight, a saponified EVA or a graft copolymer of EVA, and polycarboxylic acid anhydride. The references as identified above also disclose that the hot-melt adhesive is preferably used for adhering a metal to a metal, or a metal to a polymer having a polarity base such as polyvinyl chloride, polycarbonate, nylon, ABS resin, acrylic resin, or polyethylene terephthalate. However, the references do not refer to adhesion to a non-polar polymer such as PE.
In the event that the hot-melt adhesive contains EVA, it is not preferable to irradiate electron beams on the hot-melt adhesive because rust is likely to occur at the adhered interface to a metal.
Furthermore, when a halogen-based flame retardant, such as decabromodiphenyl ether, is added to the conventional hot-melt adhesives, e.g., the polyamide-based hot-melt adhesive, the saturated polyester-based hot-melt adhesive, the EVA-based hot-melt adhesive, the EEA-based hot-melt adhesive, to make the inner layer material flame retardant, the adhesiveness to PE deteriorates.
Under these circumstances, the development of a hot-melt adhesive that can adhere well to all PVC, a polyolefin such as PE and EVA, and a metal such as iron and copper is strongly demanded, as well as the development of a heat shrinkable tube having a flame retardant inner layer comprising a hot-melt adhesive that can adhere well to all PVC, a polyolefin such as PE and EVA, and a metal such as iron and copper.